Ion implanters are used to treat silicon wafers by bombardment of the wafers with an ion beam. One use of such beam treatment is to selectively implant the wafers with impurities of a specified dopant material, at a predetermined energy level, and in controlled concentration, to produce a semiconductor material during fabrication of a integrated circuits.
A typical ion implanter includes an ion source, an ion extraction device, a mass analysis device, a beam transport device and a wafer processing device. The ion source generates ions of desired atomic or molecular dopant species. These ions are extracted from the source by an extraction system, typically a set of electrodes, which energize and direct the flow of ions from the source, forming an ion beam. Desired ions are separated from the ion beam in a mass analysis device, typically a magnetic dipole performing mass dispersion or separation of the extracted ion beam. The beam transport device, typically a vacuum system containing a series of focusing devices, transports the ion beam to the wafer processing device while maintaining desired properties of the ion beam. Finally, semiconductor wafers are transferred in to and out of the wafer processing device via a wafer handling system, which may include one or more robotic arms, for placing a wafer to be treated in front of the ion beam and removing treated wafers from the ion implanter.
Batch-type ion implanters are well known, which typically include a spinning disk support for moving multiple silicon wafers through the ion beam. The ion beam impacts the wafer surface as the support rotates the wafers through the ion beam. Serial-type ion implanters are also known, which treat one wafer at a time. The wafers are supported in a cassette and are withdrawn one at time and placed onto a wafer support. The wafer is then oriented in an implantation orientation so that the ion beam strikes the single wafer. These serial implanters use beam shaping electronics to deflect the beam from its initial trajectory and often are used in conjunction with co-ordinated wafer support movements to selectively dope or treat the entire wafer surface. As wafers are processes through an ion implantation system they are transferred between specialized processing chambers and wafer input/output stations. Robots are routinely used to transfer wafers in to and out of the processing chamber.
Ion sources that generate the ion beams used in existing implanters are typically referred to as arc ion sources and can include heated filament cathodes for creating ions that are shaped into an appropriate ion beam for wafer treatment, U.S. Pat. No. 5,497,006 to Sferlazzo et al concerns an ion source having a cathode supported by a base and positioned with respect to a gas confinement chamber for ejecting ionizing electrons into the gas confinement chamber. The cathode of the '006 patent is a tubular conductive body having an endcap that partially extends into the gas confinement chamber. A filament is supported within the tubular body and emits electrons that heat the endcap through electron bombardment, thereby thermionically emitting ionizing electrons into the gas confinement chamber.
Extraction electrodes, as disclosed, for example, in U.S. Pat. No. 6,501,078 are generally used in conjunction with an ion source to extract a beam of ions therefrom, wherein ions formed in the confinement chamber are extracted through an exit aperture in a front face of the ion source. The front face of the ion source forms a first apertured source electrode at the potential of the ion source. The extraction electrodes typically include an apertured suppression electrode and an apertured ground electrode aligned with the first apertured source electrode (sometimes referred to as an extraction electrode) to allow the ion beam emerging from the ion source to pass therethough. Preferably, each aperture has an elongated slot configuration. Ceramic insulators are typically mounted between the suppression ad ground electrodes for electrically isolating the two electrodes. The ground electrode restricts the propogation of electric fields between the ground electrode and the ion source into the region downstream of the ground electrode. The suppression electrode is biased by a voltage supply to a negative potential relative to ground, and operates to prevent electrons in the ion beam downstream of the ground electrode from being drawn into the extraction region and into the ion source. Typically, the suppression and ground electrodes are mounted so as to be movable relatively to the source in the direction of travel of the ion beam so that the extraction electrodes can be “tuned” in accordance with the energy of the beam extracted from the ion source. The electrodes are further mounted, such that the suppression and ground electrodes are relatively laterally movable approximately perpendicular to the ion beam direction, relative to the source 20. In addition, a mechanism may also be provided for varying the size of the aperture in the electrodes.
The energy of the ion beam 30 emerging from the extraction assembly is determined by the voltage supplied to the ion source. A typical value for this voltage is 20 kV, providing extracted beam energy of 20 keV. However, extracted beam energies of 80 keV and higher, or 0.5 keV or lower may also be obtained. To obtain higher or lower beam energies, it is a matter of raising or lowering respectively the source voltage.
It has been found that the voltage biases associated with the ion source and extraction electrode system of a typical ion implantation system, in combination with the ionized source gas present in that environment leads to the formation of deposits on the suppression and ground electrodes, as well as the insulators situated therebetween. These deposits can deleteriously effect the operation of the ion implantation system, by causing decomposition of the insulators, deposits and coating of the insulators and in particular, uncontrollable release and discharge of these deposits and insulators, which create contaminating particles that are transported with the ion beam to other portions of the ion implantation system and ultimately to the workpiece being implanted.
It is an object of the present invention to provide a system for electrode Voltage modulation in an ion source extraction electrode apparatus for creating a controlled discharge therein to remove deposits in the vicinity of the electrodes and thereby mitigate contamination along the ion beamline and on the wafer in the ion implantation system. In some ways, the present invention builds on the concepts taught and disclosed in commonly assigned US Patent Application Publication No. 2011/0240889, wherein a method is provided for reducing particle contamination in an ion implantation system. In that invention, an ion implantation system a deceleration suppression plate well downstream from the ion source and ion source environment, and closely adjacent to the wafer processing end station is provided, wherein a deceleration (decel) suppression voltage applied to the decel suppression plate is modulated for causing the ion beam to expand and contract such that one or more beam line components are impacted by the ion beam to mitigate subsequent contamination of workpieces by previously deposited material residing on the surfaces of the one or more beam line components. That patent application teaches that contamination can be mitigated through voltage modulation to cause controlled beam fluctuation whereby beak strike removes previously deposited material or strongly adhering the previously deposited material to the one or more surfaces. By contrast, the present invention provides electrode voltage modulation for creating a controlled discharge between electrodes to remove deposits in the vicinity of the electrodes, thereby mitigating contamination along the ion beamline and on the wafer in the ion implantation system.